Performing upstream symbol alignment under FEXT

ABSTRACT

A method for implementing an upstream symbol alignment within a network component, the method comprising receiving an upstream sync signal via an initializing digital subscriber line (DSL) during a channel discovery phase from a customer premise equipment (CPE), determining a corrected upstream symbol alignment value based upon the upstream sync signal, and transmitting the corrected upstream symbol alignment value to the CPE, wherein the upstream symbol alignment value determines an upstream symbol alignment for one or more upstream transmissions, and wherein the corrected upstream symbol alignment value is determined before receiving a plurality of upstream data signals within the data symbol positions during the channel discovery phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application is a continuation application of U.S. Non-Provisionalapplication Ser. No. 14/249,750 filed on Apr. 10, 2014, entitled“Performing Upstream Symbol Alignment Under FEXT,” which claims benefitof U.S. Provisional Patent Application No. 61/811,334 filed Apr. 12,2013 by Haixiang Liang et al. and entitled “Upstream Symbol AlignmentProcedure Under Strong FEXT,” both of which are incorporated herein byreference as if reproduced in their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

Digital subscriber line (DSL) technologies provide a large bandwidth fordigital communications over existing subscriber lines (e.g., copperpairs). To transmit data signals, many current DSL systems, includingasymmetric DSL 2 (ADSL2), ADSL2+, very high speed DSL (VDSL), and veryhigh speed DSL 2 (VDSL2), and other DSL systems, including Fast Accessto Subscriber Terminals (G.fast), a consented standard, may employdiscrete multi-tone (DMT) modulation. Systems that perform duplextransmission using frequency division duplex (FDD), such as ADSL2 andVDSL2, separate downstream signals from the upstream signals byexchanging the signals using different frequency bands. Alternatively,systems that perform duplex transmission using time-division duplex(TDD) may use separate time intervals for upstream and downstreamtransmission. During upstream transmission from a remote side modem, forexample, there may be no downstream transmission in a G.fast DSL systemfrom a corresponding modem at a central office (CO), a fiber to thecurbside cabinet (FTTC), or distribution point unit (DPU).

When transmitting data over the subscriber lines, crosstalk interferencecan occur between the transmitted signals over adjacent lines, forexample in a same or nearby bundle of lines. Crosstalk, includingnear-end crosstalk (NEXT) and far-end crosstalk (FEXT), may limit theperformance of various DSL systems, such as those defined by standardsincluding ADSL2, VDSL, VDSL2, and G.fast. Typically, FEXT levelsincrease and become more problematic as the high frequency band edgeincreases for DSL systems. For example, a VDSL2 system may operate atbandwidth frequencies ranging from about 17 to 30 megahertz (MHz), whilea G.fast DSL system may operate at bandwidth frequencies ranging fromabout 100 MHz and higher. As such, the FEXT levels within a G.fast DSLsystem may be relatively higher (e.g. FEXT levels may be as strong asthe data signal) than a VDSL2 system.

Vectoring techniques may be used to cancel FEXT amongst subscriber lineswithin a vectored group in the downstream and upstream directions forDSL systems (e.g. VDSL2 and G.fast). Vectoring cancels crosstalk bycoordinating and managing a group of subscriber line signals in order toreduce crosstalk levels. Vectoring is described in more detail in theInternational Telecommunication Union Telecommunication StandardizationSector (ITU-T) G.993.5, entitled “Self-FEXT cancellation (vectoring) foruse with VDSL2 transceivers,” updated April 2010, which is herebyincorporated by reference as if reproduced in its entirety. In vectoredDSL systems, in order to implement downstream and upstream vectoring,the vectored DSL systems may implement symbol alignment. Specifically,downstream symbols transmitted by transceivers at the operator end(TU-Os) of a vectored group may be aligned between themselves at anoperator side interface (U-O referenced point) and upstream symbolstransmitted by transceivers at the customer end (TU-Rs) of a vectoredgroup may be aligned between themselves at the U-O reference point.Symbol alignment in the downstream direction may be achieved bytransmitting DMT symbols at the same time on all of the subscriber linesin the vectored group because the TU-Os are typically co-located, clocksynchronized, and more likely within the same DSL access multiplexer(DSLAM) equipment. Unfortunately, in comparison to the downstreamdirection, symbol alignment in the upstream direction may be relativelymore difficult because the TU-Rs are generally situated at differentlocations.

SUMMARY

In one embodiment, the disclosure includes an apparatus comprising afirst transceiver unit (TU) for coupling to a first initializing DSL, amemory, and a processor coupled to the memory and the first TU, whereinthe memory includes instructions that when executed by the processorcause the apparatus to perform the following: receive an upstream syncsignal via the first TU, determine a correction value that correspondsto an upstream symbol alignment for the first initializing DSL using theupstream sync signal, and transmit the correction value in a downstreamdirection via the first TU, wherein the correction value is determinedprior to receiving a plurality of upstream signals located in aplurality of data symbol positions.

In another embodiment, the disclosure includes a method for implementingan upstream symbol alignment within a network component, the methodcomprising receiving an upstream sync signal via an initializing DSLduring a channel discovery phase from a customer premise equipment(CPE), determining a corrected upstream symbol alignment value basedupon the upstream sync signal, and transmitting the corrected upstreamsymbol alignment value to the CPE, wherein the upstream symbol alignmentvalue determines an upstream symbol alignment for one or more upstreamtransmissions, and wherein the corrected upstream symbol alignment valueis determined before receiving a plurality of upstream data signalswithin the data symbol positions during the channel discovery phase.

In yet another embodiment, the disclosure includes an apparatuscomprising a TU for coupling to an initializing DSL, a memory, and aprocessor coupled to the memory and the first TU, wherein the memoryincludes instructions that when executed by the processor cause theapparatus to perform the following: transmit via the TU an upstream syncsignal that comprises an upstream sync symbol and a plurality of quietsymbols located within the data symbol positions, receive via the TU acorrected upstream symbol alignment value; and adjust an upstream symbolalignment for an upstream transmission via the initializing DSL based onthe corrected upstream symbol alignment value, wherein the correctedupstream symbol alignment value is received before transmitting upstreamdata signals within the data symbol positions.

In yet another embodiment, the disclosure includes a method forimplementing an upstream symbol alignment within a network component,the method comprising transmitting an upstream sync signal over aninitializing DSL during a channel discovery phase to an operator sidenode, receiving a corrected upstream symbol alignment value from theoperator side node, and transmitting a plurality of upstream signalsover the initializing DSL that are aligned based on the correctedupstream symbol alignment value to the operator side node, wherein thecorrected upstream symbol alignment value corresponds to a time gapbetween receiving and transmitting symbols by the network component, andwherein the corrected upstream symbol alignment value is determinedbefore transmitting a plurality of upstream data signals within the datasymbol positions during the channel discovery phase.

In yet another embodiment, the disclosure includes a network systemcomprising a TU-O, and a TU-R coupled to the TU-O via an initializingDSL, wherein the TU-O is configured to receive an upstream sync signalover the initializing DSL, estimate a correction value that correspondsto an upstream symbol alignment for the initializing DSL using theupstream sync signal, and transmit the correction value to the TU-R overthe initializing DSL, wherein the TU-R is configured to transmit theupstream sync signal that comprises an upstream sync symbol over theinitializing DSL, receive the correction value over the initializingDSL, adjust an upstream symbol alignment for an upstream transmissionover the initializing DSL based on the correction value, and transmitone or more upstream signals over the initializing DSL using theadjusted upstream symbol alignment, wherein the correction value isdetermined prior to receiving the upstream signals that are located in aplurality of data symbol positions at the TU-O.

In yet another embodiment, the disclosure includes a network systemcomprising a G.fast transceiver unit at an operator side (FTU-O),wherein the FTU-O is configured to perform the following during aCHANNEL DISCOVERY 1 stage for an initializing DSL: send to a G.fasttransceiver unit at remote side (FTU-R) an initial value of a time gapT_(g1′) within an O-SIGNATURE message, receive an R-P-VECTOR 1 signal,estimate a correction of the initial value of the time gap T_(g1′), andtransmit the correction of the initial value of the time gap T_(g1′) tothe FTU-R using an O-TG-UPDATE message, wherein the time gap T_(g1′) isan upstream time gap located between an end of receiving a downstreamtransmission and a start of transmitting an upstream transmission by theFTU-R.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of an xDSL system whereembodiments of the present disclosure may operate.

FIG. 2 is a schematic diagram of an embodiment of a TDD frame usedduring both initialization and showtime state for a subscriber line.

FIG. 3 is a time line diagram of an embodiment of a Channel DiscoveryPhase within an initialization procedure for a G.fast DSL system.

FIG. 4 is a time line diagram of an embodiment of an early stage of theChannel Discovery Phase for a G.fast DSL system.

FIG. 5 is a time line diagram of an embodiment of an early stage of theChannel Discovery Phase for a VDSL(2) system.

FIG. 6 is a schematic diagram of an embodiment of an O-TG-UPDATE messageused during both initialization and showtime state for a subscriberline.

FIG. 7 illustrates a flowchart of an embodiment of a method fordetermining a corrected upstream symbol alignment value for aninitializing line.

FIG. 8 illustrates a flowchart of another embodiment of a method fordetermining a corrected upstream symbol alignment value for aninitializing line.

FIG. 9 is a schematic diagram of an embodiment of a network element.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Depending on the supported standard, a DSL system may be denoted as anxDSL system, where ‘x’ may indicate any DSL standard. For instance, ‘x’stands for ‘A’ in ADSL2 or ADSL2+ systems, ‘V’ in VDSL or VDSL2 systems,and ‘F’ in G.fast systems. When a transceiver unit is located at anoperator end of the DSL system, including a CO, a DSLAM, cabinet, or aDPU, the transceiver unit may be referred to as an xTU-O. On the otherhand, when a transceiver unit is located at a remote or user end such asa CPE, the transceiver unit may be referred to as an xTU-R. For example,if the DSL system is a G.fast system, a transceiver unit at an operatorside may be referred to as a G.fast transceiver unit at an operatorside, FTU-O. Similarly, in the G.fast system, a CPE transceiver may bereferred to as a G.fast transceiver unit at remote side, FTU-R, whichmay also be referenced as a subscriber side.

Disclosed herein are at least one method, apparatus, and system thatachieve accurate upstream symbol alignment during the early stage ofinitializing a subscriber line. A control entity (CE) that comprises avector CE (VCE) may control the alignment of symbols for one or moresubscriber lines of a vectored group at a U-O reference point (e.g. U-O2reference point) and a U-R reference point (e.g. U-R2 reference point).Specifically, a CE may adjust or correct a time gap prior to usingupstream data symbol positions for training. Initially, during the earlystage of the Channel Discovery Phase, the CE may estimate an initialvalue of a time gap between receiving (e.g. downstream transmission) andtransmitting (e.g. upstream transmission) symbols by an xTU-R. Theinitial value of a time gap may be used to align transmission by anxTU-R (e.g. upstream transmission) for a joining line with active lines.The CE may subsequently adjust or correct the time gap for a subscriberline during the early stage of the Channel Discovery Phase. The xTU-Omay communicate the corrected time gap to the xTU-R in order for thexTU-R to adjust the upstream symbol alignment based on the correctedtime gap before transmitting upstream data signals within the datasymbol positions.

Throughout the disclosure, the term “initializing line” references asubscriber line that is in a state of performing the initializationprocedure, while an “active line” references a subscriber line alreadyin the showtime state. An “active line” may also be interchangeablyreferred to as a “vectored line” to signify that a subscriber line is ina showtime state and belongs to a vectored group. The terms “time gap”and “time gap correction” may also be interchangeable throughout thedisclosure with the terms “timing advance” and “timing advancecorrection,” respectively, such that “time gap” and “time gapcorrection” may be used for TDD based DSL systems and “timing advance”and “timing advance correction,” may be used for FDD based DSL systems.Additionally, for this disclosure, the term “data symbol positions”during the initialization procedure refer to symbol positions other thanthe sync symbol position within a TDD or FDD frame.

FIG. 1 is a schematic diagram of embodiment of an xDSL system 100 whereembodiments of the present disclosure may operate. The xDSL system 100may be an ADSL2, ADSL2+, VDSL, VDSL2, and/or a G.fast DSL system. ThexDSL system 100 may be configured to perform DMT, OrthogonalFrequency-Division Multiple Access (OFDMA), and/or other digitalmodulation methods. In one embodiment, the xDSL system 100 may beconfigured to perform duplex transmission using TDD (e.g. G.fast DSLsystem). In another embodiment, the xDSL system 100 may be configured toperform duplex transmission using FDD (e.g. VDSL2 system). Forconvenience purposes, throughout this disclosure, data signals that aretransmitted from the xTU-Os 1-N 104 to the xTU-Rs 1-N 112 will bereferenced as a downstream transmission, and data received by the xTU-Os1-N 104 and transmitted from the xTU-Rs 1-N 112 will be referenced as anupstream transmission.

The xDSL system 100 may comprise a DPU 118 and a plurality of CPEs 1-N122. The DPU 118 may be coupled to the CPEs 1-N 122 via a plurality ofsubscriber lines 120. The subscriber lines 120 may form transmissionspaths between the DPU 118 and the CPEs 1-N 122. The subscriber lines 120may be made of any suitable material, such as copper wire. FIG. 1 alsoillustrates a U-O reference point and a U-R reference point. The U-Oreference point may reference the operator side of the subscriber lines120 (e.g. wire pair) and the U-R reference point may reference theremote side of the subscriber lines 120. In one embodiment, the U-O2 andU-R2 reference points may be located at the same location as the U-O andU-R reference point. For example, high pass filters that are typicallyfound within splitters may be integrated within the xTU-O 104 and xTU-R112 such that the U-O2 and U-R2 reference points reference the samelocations as the U-O and U-R reference point, respectively.

The DPU 118 may comprise a physical (PHY) transmitting (Tx)/receiving(Rx) interface 110, a layer 2+ module 108, a CE 106, a management entity(ME) 102, and one or more xTU-Os 1-N 104. Each of the CPEs 1-N 122 maycomprise an xTU-R 112, a layer 2+ module 114, and a PHY Tx/Rx interface116. Each of the PHY Tx/Rx interfaces 110 and 116 may comprise aplurality of ports and a plurality of transceivers that transmit and/orreceive data signals in the electrical domain and/or in the opticaldomain. The layer 2+ modules 108 and 114 may be components configured toprocess incoming data abstracted at Open Systems Interconnection (OSI)layer 2 or higher. The ME 102 may be one or more network componentsand/or devices that provide network support information for resourceutilization and map the components within the DPU 118. For instance, ME102 may be configured to convey management information to each of thexTU-Os 1-N 104.

The CE 106 may be one or more network components and/or devices thatperform control functions and convey the operational status of the DPU118, such as identifying which subscriber line 120 to route data signalsto and obtaining the current traffic load for each subscriber line 120.In one embodiment, the CE 106 may comprise a Timing Control Entity(TCE), a VCE, and/or a Dynamic Resource Allocation (DRA) that includesthe Power Control Entity (PCE). The TCE may be configured to coordinatethe transmission and reception with synchronous time-division duplex(STDD) over a vectored group. The VCE may be configured to coordinatethe crosstalk cancellation over the vectored group. The DRA may beconfigured to coordinate the downstream and upstream transmissionopportunities over the vectored group. For example, the DRA may comprisethe PCE that may track the power consumption for users and may limit theallocation of transmission opportunities per subscriber line, in bothupstream and downstream directions. The TCE, VCE, DRA, and PCE arediscussed in more detail in ITU-T, Study Group 15, Temporary Document159 Rev. 2 (PLEN/15), and entitled “Draft Recommendation ITU-T G.9701(for AAP, 16 Jan. 2014),” January 2014, which is hereby incorporated byreference as if reproduced in its entirety. In another embodiment, theCE 106 may comprise a VCE and may not comprise the TCE when implementingFDD. The VCE may be configured to coordinate the crosstalk cancellationover the vectored group as described in ITU-T G.993.5.

FIG. 2 is a schematic diagram of an embodiment of a TDD frame 200 usedduring both initialization and showtime state for a subscriber line. Forthe downstream direction, FIG. 2 illustrates that the TDD frame T_(F1)200 may start with the FTU-O transmitting the downstream Tx symbols 202to the FTU-R. Afterwards, FTU-O may receive the upstream Rx symbols 204portion of the TDD frame T_(F1) 200 after a propagation delay of T_(pd)of transmitting the upstream Tx symbols 208 by the FTU-R. A time gapT_(g2) exists in between the end of the downstream Tx symbols 202 andthe start of the upstream Rx symbols 204 within TDD frame T_(F1) 200.Another time gap T_(g1) exists between the end of the upstream Rxsymbols 204 and the start of another downstream Tx symbols 202 locatedwithin the next TDD frame T_(F2) 200. The values of time gap T_(g1) andtime gap T_(g2) reference gaps of times at a U-O reference point (e.g.U-O interface) of the FTU-O.

For the upstream direction, FIG. 2 illustrates that for the TDD frameT_(F1) 200, the FTU-R starts to receive the downstream Rx symbols 206after a propagation delay of T_(pd) of the FTU-O transmitting thedownstream Tx symbols 202. The downstream Rx symbols 206 correspond tothe downstream Tx symbols 202 for the TDD frame T_(F1) 200. Thereceiving of downstream Rx symbols 206 is followed by the FTU-Rtransmitting the upstream Tx symbols 208 for the TDD frame T_(F1) 200 tothe FTU-O. A time gap T_(g1′) may exist between the end of thedownstream Rx symbols 206 and the start of the upstream Tx symbols 208.Another time gap T_(g2′) may exist between the end of the upstream Txsymbols 208 and the start of another downstream Rx symbols 206 for thenext TDD frame T_(F2) 200. The values of time gap T_(g1′) and T_(g2′)may reference the gap times at the U-R reference point of the FTU-R.

In G.fast DSL systems, because of the high frequency subcarriers,substantial crosstalk may exist between subscriber lines, specificallyFEXT. Without adjusting or correcting the initial value of time gapT_(g1′), the upstream symbol alignment may be a rough estimate at thestart of transmitting signals within the data symbol positions of a TDDframe. In this scenario, the transmission of data symbols withoutaccurate upstream symbol alignment (e.g. a rough upstream symbolalignment) may affect the data transmission of active lines because ofcrosstalk. Furthermore, without updating the upstream symbol alignmentat the early stages of the Channel Discovery Phase, alignment refinementand updating upstream vectoring coefficients based on the alignmentrefinement may need to be subsequently calculated again. Calculating andupdating the value of time gap T_(g1′) for upstream symbol alignmentprior to transmitting signals within the data symbol positions of a TDDframe will be discussed in more detail below.

FIG. 3 is a time line diagram of an embodiment of a Channel DiscoveryPhase 300 within an initialization procedure for a G.fast DSL system.Prior to the Channel Discovery Phase 300, the initialization procedurefor initializing lines may implement the G.994.1 Handshake Phase betweenthe FTU-O and the FTU-R. The FTU-R may initially send a request forjoining using G.994.1 handshake signals. During the G.994.1 HandshakePhase, the FTU-O and the FTU-R may exchange capabilities list, such asvectoring capabilities, and agree on a common mode for training andoperation. Further details of the G.994.1 Handshake Phase are discussedin more detail in ITU-T, G.994.1, entitled “Handshake Procedures forDigital Subscriber Line Transceivers,” June 2012, which is herebyincorporated by reference as if reproduced in its entirety. After asuccessful completion of the G.994.1 Handshake Phase, the initializationprocedure may progress to the Channel Discovery Phase 300 andsubsequently to the Channel Analysis and Exchange Phase. The ChannelAnalysis and Exchange Phase are discussed in more detail in ITU-TTemporary Document 159 Rev. 2 (PLEN/15).

FIG. 3 divides the Channel Discovery Phase 300 into a plurality ofstages that correspond to both the downstream direction and the upstreamdirection. The downstream direction corresponds to downstreamtransmission by the FTU-O, while the upstream direction refers toupstream transmission by the FTU-R. For initializing lines that completethe G.994.1 Handshake Phase, the FTU-O and the FTU-R may enter theO-QUIET stage 302 and R-QUIET stage 304, respectively. While in theO-QUIET stage 302, the FTU-O may monitor the status of the initializinglines to determine whether the initializing lines become a member of thejoining group (e.g. joining lines) or the waiting group. If the FTU-Odetermines that an initializing line is in the joining group, then theinitializing line continues to the O-VECTOR 1 stage 306. At the O-VECTOR1 stage 306, the active lines may estimate downstream crosstalk (e.g.FEXT) channels from the joining lines into active lines.

After the completion of the O-VECTOR 1 stage 306, the FTU-O enters theCHANNEL DISCOVERY 1 stage 308. During the CHANNEL DISCOVERY 1 stage 308,the FTU-O transmits O-SIGNATURE messages and O-TG-UPDATE messages forupstream symbol alignment. To perform upstream symbol alignment, theFTU-R enters the R-VECTOR 1 stage 310 after receiving the O-SIGNATUREmessage from the FTU-O. During the R-VECTOR 1 stage 310, the FTU-R maytransmit upstream sync symbols based on the time gap values received inthe O-SIGNATURE and O-TG-UPDATE message. The FTU-R may not transmitupstream signals located within the data symbol positions of a TDD frameduring R-VECTOR 1 stage 310. The O-SIGNATURE message may provide theinitial time gap values, and the O-TG-UPDATE message may provide theadjusted or corrected time gap values. After aligning upstream symboltransmission using the O-TG-UPDATE, the active lines may estimateupstream crosstalk channels from the joining lines and the joining linesmay estimate the direct channel and crosstalk channels from both theactive lines and other joining lines.

In the O-SYNCHRO stage 312, the FTU-O may send an O-P-SYNCHRO signal toindicate the end of the CHANNEL DISCOVERY 1 stage 308. The R-VECTOR 1stage 310 may end after the FTU-R receives the O-P-SYNCHRO signal.Similar to O-SYNCHRO stage 312, O-P-SYNCHRO signal may be transmitted bythe FTU-O within O-SYNCHRO stages 318, 324, and 330 to indicate the endof the CHANNEL DISCOVERY 2 stage 314, VECTOR 2 stage 320, and PARAMETERUPDATE stage 326, respectively. The FTU-R may end stages CHANNELDISCOVERY 2 stage 316, VECTOR 2 stage 322, and PARAMETER UPDATE 328after receiving the O-P-SYNCHRO signal within O-SYNCHRO stages 318, 324,and 330, respectively.

The FTU-O and the FTU-R may then transition to the CHANNEL DISCOVERY 2stage 314 and the CHANNEL DISCOVERY 2 stage 316, respectively. In theCHANNEL DISCOVERY 2 stage 316, the FTU-R may start to transmit upstreamsignals located within the data symbol positions of a TDD frame. Forexample, the FTU-R may transmit a remote-message 1 (R-MSG 1) to theFTU-O. The FTU-O, while in CHANNEL DISCOVERY 2 stage 314, may transmitan O-UPDATE message that acknowledges reception of the R-MSG 1. TheO-UPDATE message may also comprise an updated time gap value. The FTU-Rmay receive the O-UPDATE message and use the updated time gap value tofurther align the upstream symbol transmission.

The remaining stages for the Channel Discovery Phase 300, which includeVECTOR 2 stage 320, VECTOR 2 stage 322, PARAMETER UPDATE stage 326, andPARAMETER UPDATE stage 328 are discussed in more detail in ITU-TTemporary Document 159 Rev. 2 (PLEN/15). In VECTOR 2 stage 320, the CE(e.g. VCE) may perform downstream channel estimation of crosstalk fromthe active lines into the joining lines and between the joining lines.The VCE may also compute and update a variety of parameters, such asprecoder coefficients and power spectral density (PSD). The VTU-R withinthe VECTOR 2 stage 322 may transmit an error feedback message andperform downstream error estimation in order to update parameters and toperform the downstream channel estimation. During the PARAMETER UPDATEstage 326 and the PARAMETER UPDATE stage 328, the FTU-O may exchangewith the FTU-R updated transmission parameters, such as PSD andsubcarriers. The CE (e.g. VCE) may compute gains for both active linesand join lines to perform downstream spectrum optimization. At the endof the PARAMETER UPDATE stage 326 and the PARAMETER UPDATE stage 328,the PSD and transmission parameters are updated and installed for theFTU-O and the FTU-R.

FIG. 4 is a time line diagram of an embodiment of an early stage of theChannel Discovery Phase 400 for a G.fast DSL system. In the early stageof the Channel Discovery Phase 400, the FTU-O and the FTU-R may exchangeboth G.994.1 handshake signals and special operations channel (SOC)messages. The SOC channel may be established between the FTU-O and theFTU-R during initialization. During at least some of the CHANNELDISCOVERY 1 stage 308 in FIG. 3, the SOC channel may be in active statesuch that the FTU-O may transmit SOC messages that are separated by oneor more high-level data link control (HDLC) flags to the FTU-R. Themessage format of an SOC message is described in the ITU-T TemporaryDocument 159 Rev. 2 (PLEN/15).

The early stage of the Channel Discovery Phase 400 may start with theFTU-O entering the O-QUIET 1 stage 402 and the FTU-R entering theR-QUIET 1 stage 404. During the O-QUIET 1 stage 402, the FTU-O may nottransmit any data signals, and the FTU-R may not transmit any datasignals during the R-QUIET 1 stage 404. For example, the FTU-O mayproduce an O-P-QUIET 1 signal during the O-QUIET 1 stage 402 thatprovides about a zero output voltage at the U-O reference point. Also,the FTU-R may produce an R-P-QUIET 1 signal during the R-QUIET 1 stage404 that provides about a zero output voltage at the U-R referencepoint.

The FTU-O may subsequently enter the O-VECTOR 1 stage 406 whentransmitting the O-P-VECTOR 1 signal over a joining line. The O-P-VECTOR1 signal may comprise downstream sync symbols with non-zero power overthe joining line. In one embodiment, the O-P-VECTOR 1 signal may alsocomprise quiet symbols that are transported at all of the downstreamdata symbol positions. Within the O-VECTOR 1 stage 406, the active linesmay estimate downstream crosstalk channels from the joining lines intoactive lines. Specifically, the CE (e.g. VCE) may compute downstreamprecoder coefficients for active lines to cancel crosstalk from thejoining lines. The FTU-O may be configured to determine the duration ofthe O-VECTOR 1 stage 406. The FTU-R maintains the R-QUIET 1 stage 404and does not transmit any data signals to the FTU-O (e.g. upstreamtransmission) while the FTU-O is within the O-VECTOR 1 stage 406.

After the O-VECTOR 1 stage 406, the FTU-O moves to the O-ChannelDiscovery 1-1 stage 410. During the O-Channel Discovery 1-1 stage 410,the FTU-O may continue to transmit sync symbols modulated by probesequences using an O-P-Channel Discovery 1-1 signal. Within theO-Channel Discovery 1-1 stage 410, the SOC channel may be in an activestate to transmit the O-IDLE messages 408. SOC messages, such as O-IDLEmessage 408, may be transmitted over the first M downstream data symbolpositions, where M represents an integer. For example, an SOC signal maybe transmitted starting from downstream data symbol position with index0 to index 2 (e.g. M=3) of each TDD frame. Additionally, the FTU-R,which is in the R-QUIET 1 stage 404, may acquire loop timing, includingclock recovery, and symbol and TDD frame boundary alignment. The O-IDLEmessage 408 may be received by the FTU-R and used to facilitateconditions for timing recovery. The FTU-O may transition to theO-SYNCHRO 1-1 stage 412 to transmit an O-P-SYNCHRO signal to the FTU-Rto indicate the end of the O-Channel Discovery 1-1 stage 410.

The FTU-O may subsequently enter the O-Channel Discovery 1 stage 416after completing the O-SYNCHRO 1-1 stage 412. During the O-ChannelDiscovery 1 stage 416, the FTU-O continues to transmit the sync symbolsmodulated by probe sequence and also transmit SOC signals over the firstM downstream data symbols of each TDD frame. In one example embodiment,the SOC channel may transmit the O-IDLE messages 414 during eightsuperframes, followed by the transmission of the O-SIGNATURE messages418.

The O-SIGNATURE message 418 may comprise a set of parameters used foroperation of the FTU-R, such as modulation parameters, probe sequences,and initial PSD mask. In one embodiment, the O-SIGNATURE message 418 maycomprise an initial value of a time gap T_(g1′). Time gap T_(g1′) mayrepresent a time gap applied between the downstream and the upstreamtransmissions. In other words, the time gap T_(g1′) may represent thetiming offset between receiving and transmitting symbols by an FTU-R.The time gap T_(g1′) may be used to align upstream transmission of ajoining line with active lines. The initial value of time gap T_(g1′)may be calculated during the O-Channel Discovery 1 stage 416 based on afunction of loop length. For example, the initial value of time gapT_(g1′) may correspond to a maximum expected loop length for aparticular DPU. The initial value of time gap T_(g1′) may be representedas a 16-bit unsigned integer within a field in the O-SIGNATURE message418. In another embodiment, the initial timing advance value may beimplied by other information within the O-SIGNATURE message 418. Thetime gap T_(g1′) may be updated by the FTU-O during later stages ofinitialization.

During the overlap of the R-QUIET 1 stage 404 and the O-ChannelDiscovery 1 stage 416, the FTU-R may synchronize its clock with theFTU-O to achieve symbol timing and synchronization of the TDD frame. TheFTU-R may maintain transmission silence (e.g. maintain about a zerooutput voltage) within the R-QUIET 1 stage 404 until successfullydecoding the O-SIGNATURE message 418. After decoding the O-SIGNATUREmessage 418, the FTU-R may synchronize the upstream and downstream probesequences and apply parameter settings, such as the initial value oftime gap T_(g1′) obtained from O-SIGNATURE message 418. The FTU-R maythen transition to the R-VECTOR 1 stage 420 after decoding theO-SIGNATURE message 418.

Within the R-VECTOR 1 stage 420, the FTU-R transmits R-P-VECTOR 1signals that comprise upstream sync symbols modulated by a probesequence. In other words, during the R-VECTOR 1 stage 420, the FTU-Rdoes not transmit data signals located within the data symbol positionsof a TDD frame. In one embodiment, the FTU-R may transmit only upstreamsync symbols within the R-P-VECTOR 1 signals. As shown in FIG. 4, theFTU-R does not transmit SOC messages via the SOC channels. In oneembodiment, the FTU-R may transmit quiet symbols at each of the upstreamdata symbol positions for R-P-VECTOR 1 signals. The FTU-R's contents ofthe probe sequence, the time positions, and other transmissionparameters within the R-VECTOR 1 stage 420 may correspond to theinformation received in the O-SIGNATURE message 418. For instance, whentransmitting the upstream sync symbols, the FTU-R may initially alignthe upstream symbols using the initial values of time gap T_(g1′).

After the FTU-O detects the R-P-VECTOR 1 signal, the FTU-O may stoptransmitting the O-SIGNATURE messages 418 and start transmitting O-IDLEmessages 422. During the transmission of the O-IDLE messages 422, theFTU-O may be configured to estimate the correction of the initial valueof the time gap T_(g1′) based on the upstream sync symbols within theR-P-VECTOR 1 signal. For example, the FTU-O may correlate the receivedupstream sync symbols with a locally generated sync symbol, find thetime or sample differences between the peak correlation locations andthe expected locations, and use the average of the sample differences asthe correction. In one embodiment, the FTU-O may not perform upstreamFEXT channel estimation before updating the initial value of time gapT_(g1′). The FTU-O may transmit the updated value of time gap T_(g1′) tothe FTU-R in the O-TG-UPDATE message 424. The FTU-O may transmit theO-TG-UPDATE message 424 in automatic repeat (AR) mode. Once the FTU-Rreceives the O-TG-UPDATE message 424, the FTU-R may adjust upstreamsymbol alignment using the updated value of the time gap T_(g1′)received in the O-TG-UPDATE message 424.

Once the upstream symbol alignment becomes sufficiently accurate (e.g.after the FTU-R receives the update value of the time gap T_(g1′) inO-TG-UPDATE message 424), the active lines may estimate upstreamcrosstalk channels from the joining lines, and the joining lines mayestimate the direct channel and crosstalk channels from both the activelines and other joining lines. A CE (e.g. VCE) within a DPU may computethe upstream post-coder coefficients for active lines and for joininglines in order to cancel crosstalk between active lines and joininglines. The FTU-O may signal completion of the O-CHANNEL DISCOVERY 1stage 416 by sending to the FTU-R an O-P-SYNCHRO signal within O-SYNCHRO1 stage 426.

After completion of the O-CHANNEL DISCOVERY 1 stage 416, joining linesmay transmit signals within upstream data symbol positions withoutsignificantly disturbing transmission over active lines, and crosstalkfrom active lines into joining lines may be cancelled in the upstreamdirection. The Channel Discovery Phase 400 may then continue theinitializing procedure for the initializing line. For instance, theFTU-R may proceed to an R-CHANNEL DISCOVERY 2 stage and transmit theR-IDLE and the R-MSG 1 message upstream. At this point, the upstreamsignals transmitted within the R-CHANNEL DISCOVERY 2 stage may comprisedata symbols located in the data symbol positions. The value of time gapT_(g1′) may also be further updated in the late stage of the ChannelDiscovery Phase by using an O-UPDATE message. After updating the valueof time gap T_(g1′) using the O-UPDATE message, the FTU-R may use thevalue of time gap T_(g1′) to align the upstream sync symbols of theinitializing lines with the upstream sync symbols of the active lines.The upstream FEXT channel may then be re-estimated for possible changesin FEXT channels based on the new alignment of upstream sync symbols.

FIG. 5 is a time line diagram of an embodiment of an early stage of theChannel Discovery Phase 500 for a VDSL(2) system. In contrast to aG.fast DSL system, the initialization procedure for one or moreinitializing lines may further comprise a Training Phase, in addition tothe ITU-T G.994.1 Handshake Phase, Channel Discovery Phase, and ChannelAnalyze and Exchange Phase. After the Channel Discovery Phase, a VDSL2system may enter the Training Phase to further train the modems andre-estimate FEXT channels from the initializing lines into active lines.The ITU-T G.994.1 Handshake Phase, Training Phase, and Channel Analyzeand Exchange Phase are described in more detail in ITU-T G.993.5.Although the early stage of the Channel Discovery Phase 500 may beimplemented within a VDSL2 system, the details discussed below for theearly stage of the Channel Discovery Phase 500 may also be applicable toother xDSL systems, such as G.fast DSL system.

As shown in FIG. 5, the VTU-O starts out at the O-QUIET 1 stage 502 andthe VTU-R starts out at the R-QUIET 1 stage 504 at the beginning of theearly stage of the Channel Discovery Phase 500. During the O-QUIET 1stage 502 and R-QUIET 1 stage 504, both the VTU-O and the VTU-R may nottransmit data signals (e.g. output voltage is about zero). The VTU-O maythen proceed to the O-VECTOR 1 stage 506, where the CE (e.g. VCE) isused to estimate FEXT channels from the initializing lines to thevectored lines. Afterwards, the VTU-O may enter the O-CHANNEL DISCOVERYV1 stage 510 to transmit the O-SIGNATURE messages 512 to VTU-R.

The O-SIGNATURE message 512 may comprise the initial timing advance thatdefines the upstream symbol offset. The VTU-O may assign the initialtiming advance as a function of loop length. For example, the initialtiming advance value may be calculated by the FTU-O using the expectedlongest loop serviced by a vectored group. The initial timing advancevalue may be implied by other information within the O-SIGNATURE message512 or the initial timing advance value may be explicitly conveyed inthe message. An initial timing advance value may be explicitlyrepresented in the O-SIGNATURE message 512 using one of the fields tosignify the expected time offset between the downstream symbols andupstream symbols at the VTU-O's U-O interface. In one embodiment, theinitial timing advance value may be encoded within a 16-bit field usingtwo's complement format.

After the VTU-R receives and decodes the O-SIGNATURE message 512, theVTU-R transitions to the R-VECTOR 1-1 stage 516 to transmit R-P-VECTOR1-1 signals that comprise upstream sync symbols modulated by a pilotsequence. The VTU-R may transmit the R-P-VECTOR 1-1 signals such thatthe VTU-O may calculate an accurate timing advance correction. At thispoint, the VTU-O may not use the upstream sync symbols within theR-P-VECTOR 1-1 signals to perform upstream vectoring because the VTU-Omay not have determined an accurate timing advance. The active linesdata transmission may not be substantially affected by the transmissionof the upstream sync symbols. The VTU-R may transmit the upstream syncsymbols based on the initial timing advance value derived from theO-SIGNATURE message 512. With limited information from the early stageof the initialization, adjustments or corrections may need to be appliedto the initial timing advance value.

For the VTU-O to accurately estimate the upstream symbol alignmenttransmitted during the R-VECTOR 1-1 stage 516 under a FEXT environment,the upstream sync symbols within the R-P-VECTOR 1-1 signals may beselected to be unique identifiable signals for each initialization line.In other words, the upstream sync symbols may have a relatively smallcorrelation between different initialization lines. One such example isto add a constellation scrambler after the constellation encoder, andthe constellation scrambler may use different pseudo-random binarysequence (PRBS) generators for different initialization lines or thesame PRBS generator but different seed values for differentinitialization lines.

Afterwards, the VTU-R receives an O-P-SYNCHRO signal from the VTU-Owithin the O-SYNCHRO 1 stage 514, the VTU-R may stop sending upstreamsync symbols within the R-P-VECTOR 1-1 signals and enter the R-QUIET 2stage 522. The VTU-R may not perform any transmission during the R-QUIET2 stage 522. After the O-SYNCHRO 1 stage 514, the VTU-O enters theO-CHANNEL DISCOVERY 1 stage 520. The VTU-O initially transmits theO-IDLE message 518 while calculating an updated timing advance value.Once VTU-O determines an updated timing advance value, the VTU-O maystop transmitting the O-IDLE message 518 and communicate to the VTU-Rthe timing advance correction value in an O-TA-UPDATE message 524. TheO-TA-UPDATE message 524 may be sent in the same way as the O-SIGNATUREmessage 512 via the SOC channel.

Upon detecting the O-TA-UPDATE message 524, the VTU-R may start theR-VECTOR 1 stage 526 and begin transmitting the R-P-VECTOR 1 signal thatcomprises upstream sync symbols modulated by a pilot sequence using thetiming advance corrected value based on the received instruction in theO-TA-UPDATE message 524. The upstream sync symbols may be aligned withthe sync symbols of the vectored lines. During transmission withinR-P-VECTOR 1 stage 526, the CE (e.g. VCE) may estimate the FEXT channelsfrom the initializing line into all vectored lines and vice versa inorder for the VTU-Os of the vectored lines to cancel FEXT from theinitializing line. The FEXT from vectored lines to the initializing linecould also be cancelled during the R-VECTOR 1 stage 526.

To end the O-CHANNEL DISCOVERY 1 stage 520, the VTU-O may transmit anO-P-SYNCHRO signal within the O-SYNCHRO 2 stage 528. The VTU-R may endthe R-VECTOR 1 stage 526 after receiving the O-P-SYNCHRO signal. Afterthe R-VECTOR 1 stage 526, a VTU-R may start signal transmission on thedata symbol positions over the initializing line and the initializationprocedure can be continued as described in ITU-T G.993.5. For instance,the VTU-R may subsequently transmit an R-IDLE message and an R-MSG 1message via an SOC Channel within an R-CHANNEL DISCOVERY 1 stage. Boththe R-IDLE and the R-MSG 1 messages may comprise data signals on thedata symbol positions. The timing advance value may also be re-adjustedor updated in the late stage of the Channel Discovery Phase with theO-UPDATE message. Additionally, the timing advance value may be changedagain in the O-TA_UPDATE message during the Training Phase.

FIG. 6 is a schematic diagram of an embodiment of an O-TG-UPDATE message600 used during initialization for a subscriber line. The O-TG-UPDATEmessage 600 may comprise a message descriptor field 602 and a time gapcorrection (ΔT_(g1′)) field 604. The message descriptor field 602 maycomprise a message code that identifies the type of message. In oneembodiment, the O-TG-UPDATE message 600 may be about one byte long witha value of one. The time gap correction (ΔT_(g1′)) field 604 may providea correction value that represents the correction of the previouslydetermined time gap T_(g1′) (e.g. initial time gap T_(g1′) value)relative to the current time gap T_(g1′) expressed in samples (e.g. thedelta value of T_(g1′)). The time gap correction (ΔT_(g1′)) field 604may be about 16 bits long and may be encoded in two's complement format.In another embodiment, the correction value within the time gapcorrection (ΔT_(g1′)) field 604 may represent that value of the currenttime gap T_(g1′) expressed in samples. The O-TA-UPDATE message 524discussed in FIG. 5 may have a similar structure to the O-TG-UPDATEmessage 600, except that instead of a time gap correction (ΔT_(g1′))field 604, the O-TA-UPDATE message 524 may comprise a timing advancecorrection field. The timing advance correction field may besubstantially similar to the time gap correction (ΔT_(g1′)) field 604except that the correction value within the field refers to the timingadvance in FDD transmission instead of a time gap in TDD transmission.

FIG. 7 illustrates a flowchart of an embodiment of a method 700 fordetermining an upstream symbol alignment value for an initializing line.Method 700 may be implemented during the Channel Discovery Phase andprior to transmitting training signals within the data symbol positions.Method 700 may be implemented within a DPU, access node, a DSLAM, and/orany other network device on the operator side that comprises one or morexTU-Os. Transmission and receiving of data signals in the downstreamdirection may occur at the xTU-Os. Method 700 may start at block 702 anddetermine an initial upstream symbol alignment value. The initialupstream symbol alignment value may correspond to time gap T_(g1′)discussed in FIG. 4 and/or the timing advance discussed in FIG. 5.Recall that the initial upstream symbol alignment value may be afunction of a loop length.

Method 700 may then proceed to block 704 and communicate the initialupstream symbol alignment value. Method 700 may communicate the initialupstream symbol alignment value by transmitting an O-SIGNATURE messageas discussed in FIGS. 4 and 5. Method 700 may then continue to block 706and receive a uniquely identifiable signal on the upstream sync symbolposition. In one embodiment, the uniquely identifiable signal may be thesync symbol within an R-P-VECTOR 1 signal as described in FIG. 4.Another embodiment of the uniquely identifiable signal may be the syncsymbol within an R-P-VECTOR 1-1 signal as described in FIG. 5. Otherembodiments of a uniquely identifiable signal may be any other DSLsignal that comprises sync symbols with no signals located within thedata symbol positions.

Method 700 may then move to block 708 and calculate a corrected upstreamsymbol alignment value based on the sync symbol received in the uniquelyidentifiable signal. The current upstream symbol alignment value may bedetermined based on the received uniquely identifiable signal. Thecorrected upstream symbol alignment value may be the difference betweenthe initial upstream symbol alignment value and the current upstreamsymbol alignment value. In another embodiment, the corrected upstreamsymbol alignment value may be the actual value of the current upstreamsymbol alignment value. In other words, the corrected upstream symbolalignment value may correspond to the value found within the time gapcorrection (ΔT_(g1′)) field 604 (e.g. ΔT_(g1′)) as discussed in FIG. 6.After determining the corrected upstream symbol alignment value, method700 may move to block 710. At block 710, method 700 may communicate thecorrected upstream symbol alignment value. Method 700 may communicatethe correction value for the upstream symbol alignment value in anO-TG-UPDATE message 424 described in FIG. 4 and an O-TA-UPDATE message524 described in FIG. 5. Afterwards, method 700 moves to block 712 andproceeds with the remaining initialization procedures for aninitializing line.

FIG. 8 illustrates a flowchart of another embodiment of a method 800 fordetermining an upstream symbol alignment value for an initializing line.Method 800 may be implemented during the Channel Discovery Phase andprior to transmitting training signals using the data symbol positions.Method 800 may be implemented within a CPE and/or any other networkdevice on the remote side that comprises one or more xTU-Rs.Transmission and receiving of data signals in the upstream anddownstream direction, respectively, may occur at the xTU-Rs. Method 800may start at block 802 and receive an initial upstream symbol alignmentvalue from the operator side. In one embodiment, method 800 may receivethe initial upstream symbol alignment value within an O-SIGNATUREmessage as discussed in FIGS. 4 and 5. The upstream symbol alignmentvalue may be time gap T_(g1′) discussed in FIG. 4 or the timing advancediscussed in FIG. 5.

Method 800 may then continue to block 804 and communicate a uniquelyidentifiable signal located on the upstream sync symbol position basedon the received initial upstream symbol alignment value. In oneembodiment, the uniquely identifiable signal may be the sync symbolswithin an R-P-VECTOR 1 signal as described in FIG. 4. Another embodimentof the uniquely identifiable signal may be the sync symbol within anR-P-VECTOR 1-1 signal as described in FIG. 5. Other embodiments of theuniquely identifiable signal may be any other DSL signal that comprisessync symbols with no signals located within the data symbol positions.For example, the uniquely identifiable signal may consist of only syncsymbols.

Method 800 may then move to block 806 and receive a corrected upstreamsymbol alignment value (e.g. ΔT_(g1′)). The corrected upstream symbolalignment value may be received in an O-TG-UPDATE message 424 describedin FIG. 4 or an O-TA-UPDATE message 524 described in FIG. 5. Afterreceiving the correction value for the upstream symbol alignment value,method 800 may then move to block 808 and adjust the upstream symbolalignment based on the received corrected upstream symbol alignmentvalue. Afterwards, method 800 moves to block 810 and proceeds with theremaining initialization procedures for an initializing line, such astransmitting R-IDLE and R-MSG1 messages.

At least some of the features/methods described in the disclosure may beimplemented in a network element. For instance, the features/methods ofthe disclosure may be implemented using hardware, firmware, and/orsoftware installed to run on hardware. FIG. 9 is a schematic diagram ofan embodiment of a network element 900 that may be capable of receivingand transmitting DSL messages, such as user data packets that comprisedata symbols in FDD and/or TDD transmission, and state transitionrequests to and from a network, such as an xDSL system. The networkelement 900 may be any apparatus and/or network node configured toperform upstream symbol alignment. For example, network element 900 maybe a DPU, access node, or CPE within an xDSL system. The terms network“element,” “node,” “component,” “module,” and/or similar terms may beinterchangeably used to generally describe a network device and do nothave a particular or special meaning unless otherwise specificallystated and/or claimed within the disclosure.

The network element 900 may comprise one or more transceiver units 906(e.g. FTU-O and/or FTU-R), which may be transmitters, receivers, orcombinations thereof. The transceiver units 906 may transmit and/orreceive frames from other network nodes. A processor 902 may be coupledto the transceiver units 906 and may be configured to process the framesand/or determine which nodes to send (e.g. transmit) the frames. In oneembodiment, the processor 902 may comprise one or more multi-coreprocessors and/or memory modules 904, which may function as data stores,buffers, etc. The processor 902 may be implemented as a generalprocessor or may be part of one or more application specific integratedcircuits (ASICs) and/or digital signal processors (DSPs). Althoughillustrated as a single processor, the processor 902 is not so limitedand may comprise multiple processors. The processor 902 may beconfigured to implement any of the schemes described herein, includingmethods 700 and 800.

FIG. 9 illustrates that the memory module 904 may be coupled to theprocessor 902 and may be a non-transitory medium configured to storevarious types of data. Memory module 904 may comprise memory devicesincluding secondary storage, read only memory (ROM), and random accessmemory (RAM). The secondary storage is typically comprised of one ormore disk drives, solid-state drives (SSDs), and/or tape drives and isused for non-volatile storage of data and as an over-flow data storagedevice if the RAM is not large enough to hold all working data. Thesecondary storage may be used to store programs that are loaded into theRAM when such programs are selected for execution. The ROM is used tostore instructions and perhaps data that are read during programexecution. The ROM is a non-volatile memory device that typically has asmall memory capacity relative to the larger memory capacity of thesecondary storage. The RAM is used to store volatile data and perhaps tostore instructions. Access to both the ROM and the RAM is typicallyfaster than to the secondary storage.

The memory module 904 may be used to house the instructions for carryingout the system and methods described herein, e.g. method 700 implementedat DPU. In one example embodiment, the memory module 904 may comprise anupstream symbol alignment at the operator side module that may beimplemented on the processor 902. Alternately, the upstream symbolalignment at the operator side module may be implemented directly on theprocessor 902. The upstream symbol alignment at the operator side modulemay be configured to determine and calculate the initial value of thetime gap T_(g1′), update the initial value of the time gap T_(g1′) asdescribed in FIGS. 4, 5, and 7, cancel crosstalk (e.g. FEXT), and/orother functions used for upstream symbol alignment and cancel crosstalkfor an initializing line. In another embodiment, the memory module 904may comprise an upstream symbol alignment at the remote side module usedto adjust the upstream symbol alignment based on the initial value ofthe time gap T_(g1′) and the updated value of the time gap T_(g1′),and/or other functions that pertain to upstream symbol alignment. UsingFIG. 1 as an example, the CPE 122 may comprise the upstream symbolalignment at the remote side module. Examples of functions performed bythe upstream symbol alignment at the remote side module are disclosedabove in FIGS. 4, 5, and 8.

It is understood that, by programming and/or loading executableinstructions onto the network element 900, at least one of the processor902 and the memory module 904 can be changed. As a result, the networkelement 900 may be transformed in part into a particular machine orapparatus (e.g. a DPU having the functionality taught by the presentdisclosure). The executable instructions may be stored on the memorymodule 904 and loaded into the processor 902 for execution. It isfundamental to the electrical engineering and software engineering artsthat functionality that can be implemented by loading executablesoftware into a computer can be converted to a hardware implementationby well-known design rules. Decisions between implementing a concept insoftware versus hardware typically hinge on considerations of stabilityof the design and numbers of units to be produced rather than any issuesinvolved in translating from the software domain to the hardware domain.Generally, a design that is still subject to frequent change may bepreferred to be implemented in software, because re-spinning a hardwareimplementation is more expensive than re-spinning a software design.Generally, a design that is stable that will be produced in large volumemay be preferred to be implemented in hardware, for example in an ASIC,because for large production runs the hardware implementation may beless expensive than the software implementation. Often a design may bedeveloped and tested in a software form and later transformed, bywell-known design rules, to an equivalent hardware implementation in anapplication specific integrated circuit that hardwires the instructionsof the software. In the same manner, as a machine controlled by a newASIC is a particular machine or apparatus, likewise a computer that hasbeen programmed and/or loaded with executable instructions may be viewedas a particular machine or apparatus.

Any processing of the present disclosure may be implemented by causing aprocessor (e.g., a general purpose multi-core processor) to execute acomputer program. In this case, a computer program product can beprovided to a computer or a network device using any type ofnon-transitory computer readable media. The computer program product maybe stored in a non-transitory computer readable medium in the computeror the network device. Non-transitory computer readable media includeany type of tangible storage media. Examples of non-transitory computerreadable media include magnetic storage media (such as floppy disks,magnetic tapes, hard disk drives, etc.), optical magnetic storage media(e.g. magneto-optical disks), compact disc read only memory (CD-ROM),compact disc recordable (CD-R), compact disc rewritable (CD-R/W),digital versatile disc (DVD), Blu-ray (registered trademark) disc (BD),and semiconductor memories (such as mask ROM, programmable ROM (PROM),erasable PROM), flash ROM, and RAM). The computer program product mayalso be provided to a computer or a network device using any type oftransitory computer readable media. Examples of transitory computerreadable media include electric signals, optical signals, andelectromagnetic waves. Transitory computer readable media can providethe program to a computer via a wired communication line (e.g. electricwires, and optical fibers) or a wireless communication line.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations may be understood to include iterative ranges orlimitations of like magnitude falling within the expressly stated rangesor limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.;greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R_(l), and an upperlimit, R_(u), is disclosed, any number falling within the range isspecifically disclosed. In particular, the following numbers within therange are specifically disclosed: R=R_(l)+k*(R_(u)−R_(l)), wherein k isa variable ranging from 1 percent to 100 percent with a 1 percentincrement, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5percent, . . . , 50 percent, 51 percent, 52 percent, . . . , 95 percent,96 percent, 97 percent, 98 percent, 99 percent, or 100 percent.Moreover, any numerical range defined by two R numbers as defined in theabove is also specifically disclosed. The use of the term “about”means+/−10% of the subsequent number, unless otherwise stated. Use ofthe term “optionally” with respect to any element of a claim means thatthe element is required, or alternatively, the element is not required,both alternatives being within the scope of the claim. Use of broaderterms such as comprises, includes, and having may be understood toprovide support for narrower terms such as consisting of, consistingessentially of, and comprised substantially of. Accordingly, the scopeof protection is not limited by the description set out above but isdefined by the claims that follow, that scope including all equivalentsof the subject matter of the claims. Each and every claim isincorporated as further disclosure into the specification and the claimsare embodiment(s) of the present disclosure. The discussion of areference in the disclosure is not an admission that it is prior art,especially any reference that has a publication date after the prioritydate of this application. The disclosure of all patents, patentapplications, and publications cited in the disclosure are herebyincorporated by reference, to the extent that they provide exemplary,procedural, or other details supplementary to the disclosure.

While several embodiments have been provided in the present disclosure,it may be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. An apparatus comprising: a first transceiver unit(TU) for coupling to a first initializing digital subscriber line (DSL);a memory; and a processor coupled to the memory and the first TU,wherein the memory includes instructions that when executed by theprocessor cause the apparatus to perform the following: receive anupstream sync signal via the first TU in a first stage of a channeldiscovery phase; determine a correction value that corresponds to anupstream symbol alignment for the first initializing DSL using theupstream sync signal, wherein the correction value is determined usingthe upstream sync signal to correct an upstream symbol alignment of atime-division duplex (TDD) system; and transmit the correction value ina downstream direction via the first TU, determine whether the upstreamsymbol alignment of the TDD system has been completed in the first stageof the channel discovery phase; if the upstream symbol alignment of theTDD system has been completed in the first stage, transmit a signal to asecond TU coupled to the first TU over the first initialing DSL totransition to a second stage of the channel discovery phase in which thesecond TU is enabled to send a remote-message 1 (R-MSG 1) located in aplurality of data symbol positions of a TDD frame to the first TU. 2.The apparatus of claim 1, wherein the upstream sync signal comprises anupstream sync symbol modulated by a probe sequence and a plurality ofquiet symbols located within upstream data symbol positions.
 3. Theapparatus of claim 1, wherein the correction value is used to adjust atime gap between an end of receiving a downstream transmission and astart of an upstream transmission, and wherein the time gap is used toperform the upstream symbol alignment.
 4. The apparatus of claim 1,wherein the instructions, when executed by the processor, further causethe apparatus to: determine an initial value that corresponds to theupstream symbol alignment; and transmit the initial value downstream viathe first TU, wherein the correction value adjusts the initial value toupdate the upstream symbol alignment.
 5. The apparatus of claim 4,wherein the upstream sync signal is received after transmitting theinitial value downstream.
 6. The apparatus of claim 4, wherein theinitial value is transmitted within an O-SIGNATURE message during achannel discovery phase of the first initializing DSL.
 7. The apparatusof claim 1, wherein the upstream sync signal is an R-P-VECTOR 1 signalreceived in an O-CHANNEL DISCOVERY 1 stage of the first initializingDSL, and wherein the correction value is transmitted within anO-TG-UPDATE message.
 8. The apparatus of claim 1, wherein theinstructions, when executed by the processor, further cause theapparatus to not use the upstream sync signal to perform upstreamvectoring.
 9. The apparatus of claim 1, wherein the instructions, whenexecuted by the processor, further cause the apparatus to cancelcrosstalk between the first initializing DSL and a plurality of activelines and other initializing lines after transmitting the correctionvalue downstream via the first TU.
 10. The apparatus of claim 9, whereinthe instructions, when executed by the processor, further cause theapparatus to receive the upstream data signals located in the upstreamdata symbol positions after cancelling the crosstalk between the firstinitializing DSL and the active lines and other initializing lines. 11.The apparatus of claim 1, wherein the instructions, when executed by theprocessor, further cause the apparatus to: compute upstream coefficientsfor active DSL lines and for the first initializing DSL in order tocancel crosstalk between active DSL lines and the first initializing DSLafter determining that the upstream symbol alignment of the TDD systemhas been completed.
 12. A method for performing a digital subscriberline (DSL) initialization, comprising: receiving a first upstream syncsymbol associated with an upstream symbol alignment initial value via afirst DSL transceiver interface in a first stage of a channel discoveryphase of a time-division duplex (TDD) system; determining a correctionvalue of the upstream symbol alignment initial value using the firstupstream sync symbol; transmitting the correction value of the upstreamsymbol alignment initial value via the first DSL transceiver interfacein the first stage of the channel discovery phase of the TDD system;receiving a second upstream sync symbol via the first DSL transceiverinterface in response to transmission of the correction value of theupstream symbol alignment initial value; determining that an upstreamsymbol alignment of the TDD system has been completed in the first stageof the channel discovery phase of the TDD system; and after completionof the upstream symbol alignment of the TDD system in the first stage,receiving via the first DSL transceiver interface a remote-message 1(R-MSG 1) in data symbol positions of a TDD frame in a second stage ofthe channel discovery phase of the TDD system.
 13. The method of claim12, wherein the upstream symbol alignment initial value is a time gapbetween an end of receiving a downstream transmission of a single TDDframe and a start of an upstream transmission of the single TDD frame.14. The method of claim 12, further comprising: after determining thatthe upstream symbol alignment of the TDD system has been completed inthe first stage, computing upstream coefficients for active DSL linescoupled to second DSL transceiver interfaces and for a joining DSLcoupled to the first DSL transceiver interface in order to cancelcrosstalk between active DSL lines and the joining DSL.
 15. A digitalsubscriber line (DSL) transceiver comprising: a memory; and a processorcoupled to the memory, wherein the memory includes instructions thatwhen executed by the processor cause the DSL transceiver to perform thefollowing: transmit a first upstream sync symbol using an upstreamsymbol alignment initial value in a first stage of a channel discoveryphase of a time-division duplex (TDD) system; receive a correction valueof the upstream symbol alignment initial value in the first stage of thechannel discovery phase of the TDD system; transmit a second upstreamsync symbol using the correction value in the first stage of the channeldiscovery phase of the TDD system; determine whether the first stage ofthe channel discovery phase of the TDD system has been completed aftertransmitting the second upstream sync symbol using the correction,wherein in the first stage, multiple upstream sync symbols comprisingthe first upstream sync symbol using the upstream symbol alignmentinitial value and the second upstream sync symbol using the correctionvalue are transmitted to achieve upstream symbol alignment of the TDDsystem and wherein the correction value is different from the upstreamsymbol alignment initial value; and transmit a remote-message 1(R-MSG 1) in data symbol positions of a TDD frame in a second stage ofthe channel discovery phase of the TDD system if the first stage hasbeen completed.
 16. The DSL transceiver of claim 15, wherein theupstream symbol alignment initial value is a time gap between an end ofreceiving a downstream transmission of a single TDD frame and a start ofan upstream transmission of the single TDD frame.
 17. The DSLtransceiver of claim 15, wherein the processor is further to receive an0-SIGNATURE message comprising the upstream symbol alignment initialvalue in the first stage of the channel discovery phase of the TDDsystem, wherein the correction value is received via a second message inthe first stage of the channel discovery phase of the TDD system.